It is popular to set the internal parameter of an IC by an external device, for example, Linear Technology Corporation's product no. LTC6900 discloses an oscillator IC having two setting pins to be connected a resistor, a current source or a combination of a voltage source and a resistor therebetween for setting the oscillator frequency, and Intersil Corporation's product no. ISL6549 and Richtek Technology Corporation's product no. RT9259 disclose a controller IC having a setting pin to be connected to a resistor for setting the frequency of an internal oscillator in the IC. However, if the pin connected to the external parameter setting device is shorted to another pin or a hand, the resultant internal parameter may be affected and in turn lead to an error, or unstable or abnormal operation of the system using the IC. For example, in a typical power supply as shown in FIG. 1, a pulse width modulation (PWM) system uses a frequency adjustable oscillator 10 to provide a ramp signal RAMP for a PWM controller 12 to control the switching frequency of the pulse width modulation signal PWM generated by the PWM controller 12. The frequency of the oscillator 10 can be adjusted by changing a current IFS supplied to the oscillator 10, for which a reference voltage VREF is buffered by a buffer 14 to apply to a parameter setting pin FS_DIS, and the buffered reference voltage VREF and an external resistor RRT define an internal parameter I1=VREF/RRT which is mirrored by a current mirror composed of transistors M4 and M5 to generate the current IFS. Therefore, adjusting the resistance of the resistor RRT will adjust the frequency of the oscillator 10. Referring to FIG. 1 and FIG. 2, if the pin FS_DIS is shorted to another pin BOOT at time t1, for example by accumulated dust or a conductor such as water or metal therebetween, a current I2 will flow from the pin BOOT to the pin FS_DIS, causing the resistor RRT to have an increased current I3=I1+I2 and thereby increasing the voltage of the pin FS_DIS, as shown by waveform 22. Since the impedance Rs between the pins BOOT and FS_DIS is very small, the current I2 will be much larger than the current I1, causing the current IFS to decrease. Thereby, the frequency of the ramp signal RAMP is lowered, resulting in a longer period of the ramp signal RAMP lower than the feedback signal COMP, as shown by waveforms 24 and 26. The on time of the transistor M1 becomes longer because its control signal UG is postponed to turn off, as shown by waveform 28. Thus, the output voltage Vo may bounce, as shown by waveform 30, which is risky to damage the next stage. This case is worse under operation of high switching frequency, typically higher than 400 KHz and even up to 600 KHz. The customer may use smaller inductor L or smaller capacitor C in the power stage 16 to gain better transient response, so the output voltage Vo is more likely to bounce and rise faster. To date, there has been no special protection for such abnormal condition.
Although over voltage protection (OVP) may be provided to a power supply, it is almost not work because OVP needs a delay time, typically 3-10 μs, to guarantee noise immunity, and the PWM system will become crazy under this time interval. As shown by waveforms 30 and 32, during this delay time, the output voltage Vo can rise to a very high level. Although over current protection (OCP) may be provided to a power supply, the function may not work because the customer may set the OCP level higher two or more than two times of operation load. Smaller capacitor C and smaller inductor L may cause the output voltage Vo bounce fast with only several cycles, even 2-3 cycles.
Therefore, it is desired a protection apparatus and method for a parameter setting pin of an IC.